Gold 1

Gold

Gold

79Au

platinum ← gold → mercuryAg↑Au↓Rg

Gold in the periodic table

Appearance

metallic yellow

General properties

Name, symbol, number gold, Au, 79

Pronunciation /ˈɡoʊld/

Element category transition metal

Group, period, block 11, 6, d

Standard atomic weight 196.966569(5)

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Gold 2

Electron configuration [Xe] 4f14 5d10 6s1

2, 8, 18, 32, 18, 1

History

Naming aurum in Latin, meaning glow of sunrise

Discovery Middle Easterns (before 6000 BC)

Physical properties

Phase solid

Density (near r.t.) 19.30 g·cm−3

Liquid density at m.p. 17.31 g·cm−3

Melting point 1337.33 K, 1064.18 °C, 1947.52 °F

Boiling point 3129 K, 2856 °C, 5173 °F

Heat of fusion 12.55 kJ·mol−1

Heat of vaporization 324 kJ·mol−1

Molar heat capacity 25.418 J·mol−1·K−1

Vapor pressure

P (Pa) 1 10 100 1 k 10 k 100 k

at T (K) 1646 1814 2021 2281 2620 3078

Atomic properties

Oxidation states 5, 4, 3, 2, 1, −1(amphoteric oxide)

Electronegativity 2.54 (Pauling scale)

Ionization energies 1st: 890.1 kJ·mol−1

2nd: 1980 kJ·mol−1

Atomic radius 144 pm

Covalent radius 136±6 pm

Van der Waals radius 166 pm

Miscellanea

Crystal structure face centered cubic

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Gold 3

Magnetic ordering diamagnetic[1]

Electrical resistivity (20 °C) 22.14 nΩ·m

Thermal conductivity 318 W·m−1·K−1

Thermal expansion (25 °C) 14.2 µm·m−1·K−1

Speed of sound (thin rod) (r.t.) 2030 m·s−1

Tensile strength 120 MPa

Young's modulus 79 GPa

Shear modulus 27 GPa

Bulk modulus 180[citation needed] GPa

Poisson ratio 0.44

Mohs hardness 2.5

Vickers hardness 216 MPa

Brinell hardness 25 HB = ?? MPa

CAS registry number 7440-57-5

Most stable isotopes

Main article: Isotopes of gold

iso NA half-life DM DE (MeV) DP

195Au syn 186.10 d ε 0.227 195Pt

196Au syn 6.183 d ε 1.506 196Pt

β− 0.686 196Hg

197Au 100% - (α) 0.9545 193Ir

198Au syn 2.69517 d β− 1.372 198Hg

199Au syn 3.169 d β− 0.453 199Hg

Decay modes in parentheses are predicted, but have not yet been observed

Gold is a chemical element with the symbol Au and atomic number 79. It is a dense, soft, malleable, and ductilemetal with an attractive, bright yellow color and luster that is maintained without tarnishing in air or water.Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements, solidunder standard conditions. The metal therefore occurs often in free elemental (native) form, as nuggets or grains inrocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, such as withtellurium as calaverite, sylvanite and krennerite.Gold resists attacks by individual acids, but it can be dissolved by aqua regia (nitro-hydrochloric acid), so namedbecause it dissolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. Itdissolves in mercury, forming amalgam alloys; is insoluble in nitric acid, which dissolves silver and base metals, aproperty that has long been used to confirm the presence of gold in items, giving rise to the term acid test.This metal has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have sometimes been monetary policies, but were widely supplanted by fiat currency starting in the 1930s. The last gold certificate and gold coin currencies were issued in the

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Gold 4

U.S. in 1932. In Europe, most countries left the gold standard with the start of World War I in 1914 and, with hugewar debts, did not return to gold as a medium of exchange.A total of 174,100 tonnes of gold have been mined in human history, according to GFMS as of 2012.[2] This isroughly equivalent to 5.6 billion troy ounces or, in terms of volume, about 9261 m3, or a cube 21.0 m on a side. Theworld consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.Besides its widespread monetary and symbolic functions, gold has many practical uses in dentistry, electronics, andother fields. Its high malleability, ductility, resistance to corrosion and most other chemical reactions, andconductivity of electricity has led to many uses, including electric wiring, colored-glass production, and gold leafing.Most of the Earth's gold probably lies at its core, the metal's high density having made it sink there in the planet'syouth. Virtually all discovered gold is considered to have been deposited later by meteorites that contained theelement, with the asteroid that formed Vredefort crater having been implicated in the formation of the largest goldmining region on earth, Witwatersrand basin.[3]

Etymology"Gold" is cognate with similar words in many Germanic languages, deriving via Proto-Germanic *gulþą fromProto-Indo-European *gʰel- ("yellow/green").[4][5]

The symbol Au is from the Latin: aurum, according to some sources meaning "shining dawn",[6] from Sabine ausum"glowing dawn"[7] although according to definitions within Latin dictionaries the meaning of the word aurum is thesame as today's use of gold in reference to the metal.[8] The disagreement between definitions is possibly due to theaccumulation of evidence from archaeology of the original anciency of the metal in civilization; in reference to "thedawn of civilization",[9] and in this respect has become the adopted modern meaning, disassociated from the originaletymological Latin.[10]

CharacteristicsGold is the most malleable of all metals; a single gram can be beaten into a sheet of 1 square meter, or an ounce into300 square feet. Gold leaf can be beaten thin enough to become transparent. The transmitted light appears greenishblue, because gold strongly reflects yellow and red. Such semi-transparent sheets also strongly reflect infrared light,making them useful as infrared (radiant heat) shields in visors of heat-resistant suits, and in sun-visors for spacesuits.Gold readily dissolves in mercury at room temperature to form an amalgam, and forms alloys with many othermetals at higher temperatures. These alloys can be produced to modify the hardness and other metallurgicalproperties, to control melting point or to create exotic colors. Gold is a good conductor of heat and electricity andreflects infrared radiation strongly. Chemically, it is unaffected by air, moisture and most corrosive reagents, and istherefore well suited for use in coins and jewelry and as a protective coating on other, more reactive metals.However, it is not chemically inert. Gold is almost insoluble, but can be dissolved in aqua regia or solutions ofsodium or potassium cyanide, for example.Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds).Gold ions in solution are readily reduced and precipitated as metal by adding any other metal as the reducing agent.The added metal is oxidized and dissolves, allowing the gold to be displaced from solution and be recovered as asolid precipitate.In addition, gold is very dense, a cubic meter weighing 19300 kg. By comparison, the density of lead is11,340 kg/m3, and that of the densest element, osmium, is 22,588 ± 15 kg/m3.

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Gold 5

Color

Different colors of Ag-Au-Cu alloys

Whereas most other pure metals are gray or silvery white, gold isyellow. This color is determined by the density of loosely bound(valence) electrons; those electrons oscillate as a collective "plasma"medium described in terms of a quasiparticle called plasmon. Thefrequency of these oscillations lies in the ultraviolet range for mostmetals, but it falls into the visible range for gold due to subtlerelativistic effects that affect the orbitals around gold atoms. Similareffects impart a golden hue to metallic caesium.

Common colored gold alloys such as rose gold can be created by theaddition of various amounts of copper and silver, as indicated in thetriangular diagram to the left. Alloys containing palladium or nickel arealso important in commercial jewelry as these produce white gold

alloys. Less commonly, addition of manganese, aluminium, iron, indium and other elements can produce moreunusual colors of gold for various applications.

IsotopesGold has only one stable isotope, 197Au, which is also its only naturally occurring isotope. Thirty-six radioisotopeshave been synthesized ranging in atomic mass from 169 to 205. The most stable of these is 195Au with a half-life of186.1 days. The least stable is 171Au, which decays by proton emission with a half-life of 30 µs. Most of gold'sradioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, and β+ decay.The exceptions are 195Au, which decays by electron capture, and 196Au, which decays most often by electroncapture (93%) with a minor β− decay path (7%). All of gold's radioisotopes with atomic masses above 197 decay byβ− decay.At least 32 nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range,only 178Au, 180Au, 181Au, 182Au, and 188Au do not have isomers. Gold's most stable isomer is 198m2Au with ahalf-life of 2.27 days. Gold's least stable isomer is 177m2Au with a half-life of only 7 ns. 184m1Au has three decaypaths: β+ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.

Applications

Monetary exchange

Gold is commonly formed into bars for use inmonetary exchange.

Gold has been widely used throughout the world as money, forefficient indirect exchange (versus barter), and to store wealth inhoards. For exchange purposes, mints produce standardized goldbullion coins, bars and other units of fixed weight and purity.

The first coins containing gold were struck in Lydia, Asia Minor,around 600 BC. The talent coin of gold in use during the periods ofGrecian history both before and during the time of the life of Homerweighed between 8.42 and 8.75 grams. From an earlier preference inusing silver, European economies re-established the minting of gold ascoinage during the thirteenth and fourteenth centuries.

Bills (that mature into gold coin) and gold certificates (convertible into gold coin at the issuing bank) added to the circulating stock of gold standard money in most 19th century industrial economies. In preparation for World War I

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Gold 6

the warring nations moved to fractional gold standards, inflating their currencies to finance the war effort. Post-war,the victorious countries, most notably Britain, gradually restored gold-convertibility, but international flows of goldvia bills of exchange remained embargoed; international shipments were made exclusively for bilateral trades or topay war reparations.After World War II gold was replaced by a system of nominally convertible currencies related by fixed exchangerates following the Bretton Woods system. Gold standards and the direct convertibility of currencies to gold havebeen abandoned by world governments, led in 1971 by the United States' refusal to redeem its dollars in gold. Fiatcurrency now fills most monetary roles. Switzerland was the last country to tie its currency to gold; it backed 40% ofits value until the Swiss joined the International Monetary Fund in 1999.Central banks continue to keep a portion of their liquid reserves as gold in some form, and metals exchanges such asthe London Bullion Market Association still clear transactions denominated in gold, including future deliverycontracts. Today, gold mining output is declining. With the sharp growth of economies in the 20th century, andincreasing foreign exchange, the world's gold reserves and their trading market have become a small fraction of allmarkets and fixed exchange rates of currencies to gold have been replaced by floating prices for gold and gold futurecontract. Though the gold stock grows by only 1 or 2% per year, very little metal is irretrievably consumed.Inventory above ground would satisfy many decades of industrial and even artisan uses at current prices.The gold content of alloys is measured in carats (k). Pure gold is designated as 24k. English gold coins intended forcirculation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness (Americangold coins for circulation after 1837 contained the slightly lower amount of 0.900 fine gold, or 21.6 kt).Although the prices of some platinum group metals can be much higher, gold has long been considered the mostdesirable of precious metals, and its value has been used as the standard for many currencies. Gold has been used asa symbol for purity, value, royalty, and particularly roles that combine these properties. Gold as a sign of wealth andprestige was ridiculed by Thomas More in his treatise Utopia. On that imaginary island, gold is so abundant that it isused to make chains for slaves, tableware, and lavatory seats. When ambassadors from other countries arrive, dressedin ostentatious gold jewels and badges, the Utopians mistake them for menial servants, paying homage instead to themost modestly dressed of their party.

Investment

Gold prices (US$ per troy ounce), in nominal US$ and inflation adjusted US$.

Many holders of gold store it in formof bullion coins or bars as a hedgeagainst inflation or other economicdisruptions. However, economistMartin Feldstein does not believe goldserves as a hedge against inflation orcurrency depreciation.

The ISO 4217 currency code of gold isXAU.

Modern bullion coins for investment orcollector purposes do not require goodmechanical wear properties; they aretypically fine gold at 24k, although theAmerican Gold Eagle and the Britishgold sovereign continue to be minted in 22k metal in historical tradition, and the South African Krugerand, first

released in 1967, is also 22k. The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin, at 99.999% or 0.99999, while the popular issue Canadian Gold Maple Leaf coin has a purity of

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Gold 7

99.99%.Several other 99.99% pure gold coins are available. In 2006, the United States Mint began producing the AmericanBuffalo gold bullion coin with a purity of 99.99%. The Australian Gold Kangaroos were first coined in 1986 as theAustralian Gold Nugget but changed the reverse design in 1989. Other modern coins include the Austrian ViennaPhilharmonic bullion coin and the Chinese Gold Panda.

Jewelry

Moche gold necklace depicting feline heads.Larco Museum Collection. Lima-Peru

Because of the softness of pure (24k) gold, it is usually alloyed withbase metals for use in jewelry, altering its hardness and ductility,melting point, color and other properties. Alloys with lower caratrating, typically 22k, 18k, 14k or 10k, contain higher percentages ofcopper or other base metals or silver or palladium in the alloy. Copperis the most commonly used base metal, yielding a redder color.[11]

Eighteen-carat gold containing 25% copper is found in antique andRussian jewelry and has a distinct, though not dominant, copper cast,creating rose gold. Fourteen-carat gold-copper alloy is nearly identicalin color to certain bronze alloys, and both may be used to producepolice and other badges. Blue gold can be made by alloying with ironand purple gold can be made by alloying with aluminium, although rarely done except in specialized jewelry. Bluegold is more brittle and therefore more difficult to work with when making jewelry.

Fourteen- and eighteen-carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold.White gold alloys can be made with palladium or nickel. White 18-carat gold containing 17.3% nickel, 5.5% zincand 2.2% copper is silvery in appearance. Nickel is toxic, however, and its release from nickel white gold iscontrolled by legislation in Europe.Alternative white gold alloys are available based on palladium, silver and other white metals, but the palladiumalloys are more expensive than those using nickel. High-carat white gold alloys are far more resistant to corrosionthan are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the color contrasts betweenlaminated colored gold alloys to produce decorative wood-grain effects.

MedicineGold is perhaps the most anciently administered medicine (apparently by shamanic practitioners) and known toDioscorides,[12] apparent paradoxes of the actual toxicology of the substance nevertheless suggests the possibilitystill of serious gaps in understanding of action on physiology.In medieval times, gold was often seen as beneficial for the health, in the belief that something so rare and beautifulcould not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallicgold a healing power. Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in thetreatment of arthritis and other similar conditions. Gold based injections have been explored as a means to help toreduce the pain and swelling of rheumatoid arthritis and tuberculosis. However, only salts and radioisotopes of goldare of pharmacological value, as elemental (metallic) gold is inert to all chemicals it encounters inside the body.Gold alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges.The gold alloys' slight malleability facilitates the creation of a superior molar mating surface with other teeth andproduces results that are generally more satisfactory than those produced by the creation of porcelain crowns. Theuse of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.Colloidal gold preparations (suspensions of gold nanoparticles) in water are intensely red-colored, and can be made with tightly controlled particle sizes up to a few tens of nanometers across by reduction of gold chloride with citrate

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Gold 8

or ascorbate ions. Colloidal gold is used in research applications in medicine, biology and materials science. Thetechnique of immunogold labeling exploits the ability of the gold particles to adsorb protein molecules onto theirsurfaces. Colloidal gold particles coated with specific antibodies can be used as probes for the presence and positionof antigens on the surfaces of cells. In ultrathin sections of tissues viewed by electron microscopy, the immunogoldlabels appear as extremely dense round spots at the position of the antigen.Gold, or alloys of gold and palladium, are applied as conductive coating to biological specimens and othernon-conducting materials such as plastics and glass to be viewed in a scanning electron microscope. The coating,which is usually applied by sputtering with an argon plasma, has a triple role in this application. Gold's very highelectrical conductivity drains electrical charge to earth, and its very high density provides stopping power forelectrons in the electron beam, helping to limit the depth to which the electron beam penetrates the specimen. Thisimproves definition of the position and topography of the specimen surface and increases the spatial resolution of theimage. Gold also produces a high output of secondary electrons when irradiated by an electron beam, and theselow-energy electrons are the most commonly used signal source used in the scanning electron microscope.The isotope gold-198 (half-life 2.7 days) is used, in nuclear medicine, in some cancer treatments and for treatingother diseases.

Food and drink• Gold can be used in food and has the E number 175.• Gold leaf, flake or dust is used on and in some gourmet foods, notably sweets and drinks as decorative ingredient.

Gold flake was used by the nobility in medieval Europe as a decoration in food and drinks, in the form of leaf,flakes or dust, either to demonstrate the host's wealth or in the belief that something that valuable and rare must bebeneficial for one's health.

• Danziger Goldwasser (German: Gold water of Danzig) or Goldwasser (English: Goldwater) is a traditionalGerman herbal liqueur produced in what is today Gdańsk, Poland, and Schwabach, Germany, and contains flakesof gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf.[13] However,since metallic gold is inert to all body chemistry, it has no taste, it provides no nutrition, and it leaves the bodyunaltered.

Industry

Mirror for the future James Webb SpaceTelescope coated in gold to reflect infrared light

• Gold solder is used for joining the components of gold jewelry byhigh-temperature hard soldering or brazing. If the work is to be ofhallmarking quality, gold solder must match the carat weight of thework, and alloy formulas are manufactured in mostindustry-standard carat weights to color match yellow and whitegold. Gold solder is usually made in at least three melting-pointranges referred to as Easy, Medium and Hard. By using the hard,high-melting point solder first, followed by solders withprogressively lower melting points, goldsmiths can assemblecomplex items with several separate soldered joints.

• Gold can be made into thread and used in embroidery.• Gold produces a deep, intense red color when used as a coloring agent in cranberry glass.• In photography, gold toners are used to shift the color of silver bromide black-and-white prints towards brown or

blue tones, or to

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Gold 9

The world's largest gold bar has amass of 250 kg. Toi museum, Japan.

A gold nugget of 5 mm in diameter(bottom) can be expanded through

hammering into a gold foil of about0.5 square meters. Toi museum,

Japan.

increase their stability. Used on sepia-toned prints, gold toners produce redtones. Kodak published formulas for several types of gold toners, which usegold as the chloride.[14]

• Gold is a good reflector of electromagnetic radiation such as infrared andvisible light as well as radio waves. It is used for the protective coatings onmany artificial satellites, in infrared protective faceplates in thermal protectionsuits and astronauts' helmets and in electronic warfare planes like the EA-6BProwler.

• Gold is used as the reflective layer on some high-end CDs.• Automobiles may use gold for heat shielding. McLaren uses gold foil in the

engine compartment of its F1 model.• Gold can be manufactured so thin that it appears transparent. It is used in

some aircraft cockpit windows for de-icing or anti-icing by passing electricitythrough it. The heat produced by the resistance of the gold is enough to deterice from forming.

Electronics

The concentration of free electrons in gold metal is 5.90×1022 cm−3. Gold ishighly conductive to electricity, and has been used for electrical wiring in somehigh-energy applications (only silver and copper are more conductive pervolume, but gold has the advantage of corrosion resistance). For example, goldelectrical wires were used during some of the Manhattan Project's atomicexperiments, but large high current silver wires were used in the calutron isotopeseparator magnets in the project.

Though gold is attacked by free chlorine, its good conductivity and generalresistance to oxidation and corrosion in other environments (including resistanceto non-chlorinated acids) has led to its widespread industrial use in the electronicera as a thin layer coating electrical connectors, thereby ensuring goodconnection. For example, gold is used in the connectors of the more expensiveelectronics cables, such as audio, video and USB cables. The benefit of usinggold over other connector metals such as tin in these applications has been debated; gold connectors are oftencriticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy. However,the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and inuse for contacts with a very high failure cost (certain computers, communications equipment, spacecraft, jet aircraftengines) remains very common.

Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion,electrical conductivity, ductility and lack of toxicity. Switch contacts are generally subjected to more intensecorrosion stress than are sliding contacts. Fine gold wires are used to connect semiconductor devices to theirpackages through a process known as wire bonding.

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Gold 10

Commercial chemistryGold is attacked by and dissolves in alkaline solutions of potassium or sodium cyanide, to form the salt goldcyanide—a technique that has been used in extracting metallic gold from ores in the cyanide process. Gold cyanideis the electrolyte used in commercial electroplating of gold onto base metals and electroforming.Gold chloride (chloroauric acid) solutions are used to make colloidal gold by reduction with citrate or ascorbate ions.Gold chloride and gold oxide are used to make cranberry or red-colored glass, which, like colloidal goldsuspensions, contains evenly sized spherical gold nanoparticles.

Cultural history

The Turin Papyrus Map

Funerary mask of Tutankhamun

Gold artifacts found at the Nahal Kana cave cemetery dated during the1980s, showed these to be from within the Chalcolithic, and consideredthe earliest find from the Levant (Gopher et al. 1990). Gold artifacts inthe Balkans also appear from the 4th millennium BC, such as thosefound in the Varna Necropolis near Lake Varna in Bulgaria, thought byone source (La Niece 2009) to be the earliest "well-dated" find of goldartifacts. Gold artifacts such as the golden hats and the Nebra diskappeared in Central Europe from the 2nd millennium BC Bronze Age.

Egyptian hieroglyphs from as early as 2600 BC describe gold, whichking Tushratta of the Mitanni claimed was "more plentiful than dirt" inEgypt. Egypt and especially Nubia had the resources to make themmajor gold-producing areas for much of history. The earliest knownmap is known as the Turin Papyrus Map and shows the plan of a goldmine in Nubia together with indications of the local geology. Theprimitive working methods are described by both Strabo and DiodorusSiculus, and included fire-setting. Large mines were also present acrossthe Red Sea in what is now Saudi Arabia.

The legend of the golden fleece may refer to the use of fleeces to trapgold dust from placer deposits in the ancient world. Gold is mentionedfrequently in the Old Testament, starting with Genesis 2:11 (atHavilah), the story of The Golden Calf and many parts of the templeincluding the Menorah and the golden altar. In the New Testament, it is included with the gifts of the magi in thefirst chapters of Matthew. The Book of Revelation 21:21 describes the city of New Jerusalem as having streets"made of pure gold, clear as crystal". Exploitation of gold in the south-east corner of the Black Sea is said to datefrom the time of Midas, and this gold was important in the establishment of what is probably the world's earliestcoinage in Lydia around 610 BC. From the 6th or 5th century BC, the Chu (state) circulated the Ying Yuan, one kindof square gold coin.

In Roman metallurgy, new methods for extracting gold on a large scale were developed by introducing hydraulicmining methods, especially in Hispania from 25 BC onwards and in Dacia from 106 AD onwards. One of theirlargest mines was at Las Medulas in León (Spain), where

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Gold 11

Jason returns with the golden fleeceon an Apulian red-figure calyx

krater, ca. 340–330 BC.

Ancient Greek golden decorated crown, funeraryor marriage material, 370–360 BC. From a grave

in Armento, Campania

seven long aqueducts enabled them to sluice most of a large alluvialdeposit. The mines at Roşia Montană in Transylvania were also verylarge, and until very recently, still mined by opencast methods. Theyalso exploited smaller deposits in Britain, such as placer and hard-rockdeposits at Dolaucothi. The various methods they used are welldescribed by Pliny the Elder in his encyclopedia Naturalis Historiawritten towards the end of the first century AD.

During Mansa Musa's (ruler of the Mali Empire from 1312 to 1337)hajj to Mecca in 1324, he passed through Cairo in July 1324, and wasreportedly accompanied by a camel train that included thousands ofpeople and nearly a hundred camels where he gave away so much goldthat it depressed the price in Egypt for over a decade.[15] Acontemporary Arab historian remarked:

Gold was at a high price in Egypt until they came in that year.The mithqal did not go below 25 dirhams and was generallyabove, but from that time its value fell and it cheapened in priceand has remained cheap till now. The mithqal does not exceed22 dirhams or less. This has been the state of affairs for abouttwelve years until this day by reason of the large amount of goldwhich they brought into Egypt and spent there [...]—Chihab Al-Umari,

The Portuguese overseas expansion started in 1415 with the taking ofCeuta, to control the gold trade coming across the desert. Although thecaravan trade routes were then diverted, the Portuguese continuedexpansing southwards along the coast and eventually buying the golddirectly (or less indirectly) from the Africans in the Gulf ofGuinea.[citation needed]

The European exploration of the Americas was fueled in no small partby reports of the gold ornaments displayed in great profusion by NativeAmerican peoples, especially in Central America, Peru, Ecuador andColombia. The Aztecs regarded gold as literally the product of thegods, calling it "god excrement" (teocuitlatl in Nahuatl), and afterMontezuma was killed, most of this gold was shipped to Spain.However, for the indigenous peoples of North America gold wasconsidered useless and they saw much greater value in other mineralswhich were directly related to their utility, such as obsidian, flint, and slate.[16]

Gold played a role in western culture, as a cause for desire and of corruption, as told in children's fables likeRumplestiltskin, where the peasant's daughter turns hay into gold, in return for giving up her child when shebecomes a princess, and stealing the hen that lays golden eggs in Jack and the beanstalk.

The top prize at the Olympic games is the gold medal.There is an age-old tradition of biting gold to test its authenticity. Although this is certainly not a professional way of examining gold, the bite test was not to check if the coin was gold (90% gold coins are fairly strong) but to see if the coin was gold plated lead. A lead coin would be very soft and thus teeth marks would result. Fake gold coins were a common problem before 1932 so weighing a coin and also sliding a coin through a "counterfeit detector" slot was common (making a lead coin thicker would add weight thus why slide it through a measured slot). Most

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Gold 12

establishments (especially US Western saloons) would never accept a gold (or silver) coin of high value beforeweighing such an item.[citation needed]

75% of all gold ever produced has been extracted since 1910. It has been estimated that all gold ever refined wouldform a single cube 20 m (66 ft) on a side (equivalent to 8,000 m3).One main goal of the alchemists was to produce gold from other substances, such as lead — presumably by theinteraction with a mythical substance called the philosopher's stone. Although they never succeeded in this attempt,the alchemists promoted an interest in what can be done with substances, and this laid a foundation for today'schemistry. Their symbol for gold was the circle with a point at its center (☉), which was also the astrological symboland the ancient Chinese character for the Sun.Golden treasures have been rumored to be found at various locations, following tragedies such as the Jewish templetreasures in the Vatican, following the temple's destruction in 70 AD, a gold stash on the Titanic, the Nazi gold train– following World War II.The Dome of the Rock on the Jerusalem temple site is covered with an ultra-thin golden glasureWikipedia:Pleaseclarify. The Sikh Golden temple, the Harmandir Sahib, is a building covered with gold. Similarly the Wat Phra Kaewemerald Budha temple in Thailand has ornamental gold statues walls and roofs. Some European king and queen'scrowns were made of gold, and gold was used for the bridal crown since antiquity. An ancient Talmudic text circa100 AD describes Rachel, Rabbi Akiba's wife asking for a "Jerusalem of Gold" (crown). A Greek burial crown madeof gold was found in a grave circa 370 BC.

Occurrence

This 156-troy-ounce (4.9 kg) nugget, known asthe Mojave Nugget, was found by an individual

prospector in the Southern California Desertusing a metal detector.

Gold's atomic number of 79 makes it one of the higher atomic numberelements which occur naturally. Like all elements with atomic numberslarger than iron, gold is thought to have been formed from a supernovanucleosynthesis process,[citation needed] although a newer theorysuggests they are made by the collision of neutron stars instead. Eitherway, satellites should be able to detect the resulting gold, "but we haveno spectroscopic evidence that [such] elements have truly beenproduced." [17] These theories hold that the resulting explosionsscattered metal-containing dusts (including heavy elements like gold)into the region of space in which they later condensed into our solarsystem and the Earth. Because the Earth was molten when it was justformed, almost all of the gold present on Earth sank into the core. Mostof the gold that is present today in the Earth's crust and mantle wasdelivered to Earth by asteroid impacts during the late heavybombardment.

On Earth, gold is found in ores in rock formed from the Precambrian time onward. It most often occurs as a nativemetal, typically in a metal solid solution with silver (i.e. as a gold silver alloy). Such alloys usually have a silvercontent of 8–10%. Electrum is elemental gold with more than 20% silver. Electrum's color runs from golden-silveryto silvery, dependent upon the silver content. The more silver, the lower the specific gravity.

Native gold occurs as very small to microscopic particles embedded in rock, often together with quartz or sulfideminerals such as "Fool's Gold", which is a pyrite. These are called lode deposits. The metal in a native state is alsofound in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvialdeposits called placer deposits. Such free gold is always richer at the surface of gold-bearing veinsWikipedia:Pleaseclarify owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust intostreams and rivers, where it collects and can be welded by water action to form nuggets.

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Gold 13

Relative sizes of an 860 kg block of gold ore, andthe 30 g of gold that can be extracted from it. Toi

gold mine, Japan.

Gold left behind after a pyrite cube was oxidizedto hematite. Note cubic shape of cavity.

Gold sometimes occurs combined with tellurium as the mineralscalaverite, krennerite, nagyagite, petzite and sylvanite (see tellurideminerals), and as the rare bismuthide maldonite (Au2Bi) andantimonide aurostibite (AuSb2). Gold also occurs in rare alloys withcopper, lead, and mercury: the minerals auricupride (Cu3Au),novodneprite (AuPb3) and weishanite ((Au, Ag)3Hg2).

Recent research suggests that microbes can sometimes play animportant role in forming gold deposits, transporting and precipitatinggold to form grains and nuggets that collect in alluvial deposits.[18]

Another recent study has claimed water in faults vaporizes during anearthquake, depositing gold. When an earthquake strikes, it movesalong a fault. Water often lubricates faults, filling in fractures and jogs.About 6 miles (10 kilometers) below the surface, under incredibletemperatures and pressures, the water carries high concentrations ofcarbon dioxide, silica, and gold. During an earthquake, the fault jogsuddenly opens wider. The water inside the void instantly vaporizes,flashing to steam and forcing silica, which forms the mineral quartz,and gold out of the fluids and onto nearby surfaces.

Seawater

The world's oceans contain gold. Measured concentrations of gold inthe Atlantic and Northeast Pacific are 50–150 fmol/L or 10–30 partsper 1,000,000,000,000,000 quadrillion (about 10–30 g/km3). Ingeneral, gold concentrations for south Atlantic and central Pacificsamples are the same (~50 fmol/L) but less certain. Mediterranean deep waters contain slightly higher concentrationsof gold (100–150 fmol/L) attributed to wind-blown dust and/or rivers. At 10 parts per quadrillion the Earth's oceanswould hold 15,000 tonnes of gold. These figures are three orders of magnitude less than reported in the literatureprior to 1988, indicating contamination problems with the earlier data.

A number of people have claimed to be able to economically recover gold from sea water, but so far they have allbeen either mistaken or acted in an intentional deception. Prescott Jernegan ran a gold-from-seawater swindle in theUnited States in the 1890s. A British fraudster ran the same scam in England in the early 1900s.[19] Fritz Haber (theGerman inventor of the Haber process) did research on the extraction of gold from sea water in an effort to help payGermany's reparations following World War I. Based on the published values of 2 to 64 ppb of gold in seawater acommercially successful extraction seemed possible. After analysis of 4,000 water samples yielding an average of0.004 ppb it became clear that the extraction would not be possible and he stopped the project. No commerciallyviable mechanism for performing gold extraction from sea water has yet been identified. Gold synthesis is noteconomically viable and is unlikely to become so in the foreseeable future.

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Gold 14

Specimens of crystalline native gold

Native gold nuggets "Rope gold" fromLena River, SakhaRepublic, Russia.

Size: 2.5×1.2×0.7 cm.

Crystalline gold from MinaZapata, Santa Elena de Uairen,

Venezuela. Size:3.7×1.1×0.4 cm.

Gold leaf fromHarvard Mine,

Jamestown,California, USA.

Size 9.3×3.2×>0.1 cm.

Production

The entrance to an underground gold mine inVictoria, Australia

Pure gold precipitate produced by the aqua regiarefining process

At the end of 2009, it was estimated that all the gold ever mined totaled165,000 tonnes. This can be represented by a cube with an edge lengthof about 20.28 meters. At $1,600 per troy ounce, 165,000 metrictonnes of gold would have a value of $8.5 trillion.

World production for 2011 was at 2,700 tonnes, compared to 2,260tonnes for 2008.

Since the 1880s, South Africa has been the source for a largeproportion of the world's gold supply, with about 50% of all gold everproduced having come from South Africa. Production in 1970accounted for 79% of the world supply, producing about 1,480 tonnes.In 2007 China (with 276 tonnes) overtook South Africa as the world'slargest gold producer, the first time since 1905 that South Africa hasnot been the largest.

Mining

The city of Johannesburg located in South Africa was founded as aresult of the Witwatersrand Gold Rush which resulted in the discoveryof some of the largest gold deposits the world has ever seen. Goldfields located within the basin in the Free State and Gauteng provincesare extensive in strike and dip requiring some of the world's deepestmines, with the Savuka and TauTona mines being currently the world'sdeepest gold mine at 3,777 m. The Second Boer War of 1899–1901between the British Empire and the Afrikaner Boers was at least partlyover the rights of miners and possession of the gold wealth in South Africa.

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Gold 15

Time trend of gold production

Other major producers are the United States, Australia, Russia, andPeru, as well as, Ghana, Burkina Faso, Mali, Indonesia andUzbekistan. Mines in South Dakota and Nevada supply two-thirds ofgold used in the United States. In South America, the controversialproject Pascua Lama aims at exploitation of rich fields in the highmountains of Atacama Desert, at the border between Chile andArgentina. Today about one-quarter of the world gold output isestimated to originate from artisanal or small scale mining.

Prospecting

During the 19th century, gold rushes occurred whenever large gold deposits were discovered. The first documenteddiscovery of gold in the United States was at the Reed Gold Mine near Georgeville, North Carolina in 1803. The firstmajor gold strike in the United States occurred in a small north Georgia town called Dahlonega. Further gold rushesoccurred in California, Colorado, the Black Hills, Otago in New Zealand, Australia, Witwatersrand in South Africa,and the Klondike in Canada.

ExtractionGold extraction is most economical in large, easily mined deposits. Ore grades as little as 0.5 mg/kg (0.5 parts permillion, ppm) can be economical. Typical ore grades in open-pit mines are 1–5 mg/kg (1–5 ppm); ore grades inunderground or hard rock mines are usually at least 3 mg/kg (3 ppm). Because ore grades of 30 mg/kg (30 ppm) areusually needed before gold is visible to the naked eye, in most gold mines the gold is invisible.The average gold mining and extraction costs were about US$317/oz in 2007, but these can vary widely dependingon mining type and ore quality; global mine production amounted to 2,471.1 tonnes.

RefiningAfter initial production, gold is often subsequently refined industrially by the Wohlwill process which is based onelectrolysis or by the Miller process, that is chlorination in the melt. The Wohlwill process results in higher purity,but is more complex and is only applied in small-scale installations. Other methods of assaying and purifying smalleramounts of gold include parting and inquartation as well as cupellation, or refining methods based on the dissolutionof gold in aqua regia.

Synthesis from other elementsGold was synthesized from mercury by neutron bombardment in 1941, but the isotopes of gold produced were allradioactive. In 1924, a Japanese physicist, Hantaro Nagaoka, accomplished the same feat.Gold can currently be manufactured in a nuclear reactor by irradiation either of platinum or mercury.Only the mercury isotope 196Hg, which occurs with a frequency of 0.15% in natural mercury, can be converted togold by neutron capture, and following electron capture-decay into 197Au with slow neutrons. Other mercuryisotopes are converted when irradiated with slow neutrons into one another, or formed mercury isotopes which betadecay into thallium.Using fast neutrons, the mercury isotope 198Hg, which composes 9.97% of natural mercury, can be converted bysplitting off a neutron and becoming 197Hg, which then disintegrates to stable gold. This reaction, however,possesses a smaller activation cross-section and is feasible only with un-moderated reactors.It is also possible to eject several neutrons with very high energy into the other mercury isotopes in order to form197Hg. However such high-energy neutrons can be produced only by particle accelerators.Wikipedia:Please clarify.

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Gold 16

ConsumptionThe consumption of gold produced in the world is about 50% in jewelry, 40% in investments, and 10% inindustry.[citation needed]

India is the world's largest single consumer of gold, as Indians buy about 25% of the world's gold, purchasingapproximately 800 tonnes of gold every year, mostly for jewelry. India is also the largest importer of gold; in 2008,India imported around 400 tonnes of gold. Indian households hold 18,000 tonnes of gold which represents 11% ofthe global stock and worth more than $950 billion.

Gold jewelry consumption by country in tonnes

Country 2009 2010 2011 2012

 India 442.37 745.70 986.3 864

Greater China 376.96 428.00 921.5 817.5

 United States 150.28 128.61 199.5 161

 Turkey 75.16 74.07 143 118

 Saudi Arabia 77.75 72.95 69.1 58.5

 Russia 60.12 67.50 76.7 81.9

 United Arab Emirates 67.60 63.37 60.9 58.1

 Egypt 56.68 53.43 36 47.8

 Indonesia 41.00 32.75 55 52.3

 United Kingdom 31.75 27.35 22.6 21.1

Other Persian Gulf Countries 24.10 21.97 22 19.9

 Japan 21.85 18.50 −30.1 7.6

 South Korea 18.83 15.87 15.5 12.1

 Vietnam 15.08 14.36 100.8 77

 Thailand 7.33 6.28 107.4 80.9

Total 1508.70 1805.60

Other Countries 251.6 254.0 390.4 393.5

World Total 1760.3 2059.6 3487.5 3163.6

PollutionGold production is associated with contribution to hazardous pollution.[20] The ore, generally containing less thanone ppm gold metal, is ground and mixed with sodium cyanide or mercury to react with gold in the ore for goldseparation. Cyanide is a highly poisonous chemical, which can kill living creatures when exposed in minutequantities. Many cyanide spills[21] from gold mines have occurred in both developed and developing countries whichkilled marine life in long stretches of affected rivers. Environmentalists consider these events major environmentaldisasters.[22][23] When mercury is used in gold production, minute quantity of mercury compounds reach waterbodies, causing heavy metal contamination. Mercury can then enter into the human food chain in the form of methylmercury. Mercury poisoning in humans causes incurable brain function damage and severe retardation.Thirty tonnes of used ore is dumped as waste for producing one 1 ounce (28 g) of gold.[24] Gold ore dumps are the source of many heavy elements such as cadmium, lead, zinc, copper, arsenic, selenium and mercury. When sulfide bearing minerals in these ore dumps are exposed to air and water, the sulfide transforms into sulfuric acid which in

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